Following on from this post the van has run fine for a week with no more high temp issues, then yesterday it developed a misfire/splutter on the motorway which concerned me. Fortunately the courier firm I was working for yesterday had one of their Astras free so I left my van at their yard and used their van to finish the job. When I picked my van up it ran fine on the way home but I decided to get a fresh fuel filter as I've racked about 27,000 miles up since it was last changed.

It was only after I changed the filter I thought I'd check the coolant and thats when I discovered oil in the header tank. The tank doesn't look overly dirty from the outside but I dipped a paper towel in and it's quite thick. This may also explain why my oil level indicator has been a bit erratic over the last couple of days.

Now my question is how would oil and coolant mix? I've got a horrible feeling the answer iss going to be a failed head gasket given the high temp symptoms I had the other week

edit: The van had the oil cooler replaced when I bought it (about 85,000 miles). It now has 196,000 miles on it. Is the cooler a known weak spot on this engine? It's the only other place I can think of where water and oil could mix. As I need it sorting ASAP I think it's gonna be a trip to a garage :-S

Regarding your other post, I had the opportunity to study the cooling system on my 1.6hdi whilst sitting in standing traffic yesterday. My van usually runs at 80deg, but whilst sitting in standing traffic it slowly crept up past 90deg and the fans eventually came on as the needle reached the next mark which I think is 100deg and it dropped right back to 90deg. On the occasions that the van did move the air flow through the radiator bought the temperature down noticeably. I would not expect the temperature to get much above 90deg when driving unless you were driving up long hills with a lot of weight on and would only expect the fans to come on when stationary or moving very slowly.
If you had an oil and water mix I would expect emulsified oil (mayonnaise) on the inside of your oil filler cap, but not always. Leaking combustion gasses can leave an oil like deposit.
A garage can test your coolant to identify any contamination, but with the missfire it does sound as if you've lost compression on one or more cylinders.
When I did my head gasket it cost quite a lot in parts alone, I had the head skimmed which included re-cutting 16 valves, a new set of head bolts and a gasket set which came to around £280 and I also replaced the cam belt. (I got the head done at Blackburn Brothers in Preston 01772 822703 and they also supplied the gasket set and bolts.) Labour from a garage would put a big chunk on top of that as it is not a five minute job to get the head off and if the injector seals have been blowing it can be a real tw@ of a job to remove the injectors. The head won't come off without them coming out first.
As a tip if you do the job yourself; the manual says that you have to remove the fuel pump at the back of the engine, you don't, there is a short stud that needs to be removed where it bolts to the head.
The manual also says that you should not re-use the high pressure injector pipes, I did without any problems, just unscrew them from the injectors and bend them up a bit to make room.
Also mark up your injectors as they have to go back in the same cylinder.
Good luck.

I'm keeping my fingers crossed it's not the head gasket. Other than the splutter (misfire might be an exagerated way of describing what it did) it behaved normally all of this week with temp never going above 90 even when stationary or going up hills, and I recon I've done over 2,000 miles this week.

With 195,000 miles on the clock it may not be worth chucking loads of money at it if it turns out to be a head gasket problem even though it is in otherwise in good nick. I certainly wouldn't tackle a head gasket change myself, oil cooler looks simple to change but I'll probably just leave it to the garage. At least as it's a vehicle for my business I can offset the costs as a business expense.